The Siemens brochures "Halbleiterbeispiele", 1964, pages 44 and 45, and 1966, pages 49 to 52, disclose circuits in which the signal provided by a photoelectric cell and amplified is compared with a fixed reference voltage by means of a Schmitt trigger or a comparator. When the signal exceeds or falls below the reference voltage, a corresponding logic signal will be produced which serves to indicate the presence or absence of the medium to be detected. These circuits have the disadvantage of also responding to signal variations not emanating from the medium to be detected, and this results in misinterpretations. Since such signal variations, caused by hum, temperature changes, component aging, contamination, supply-voltage variations, and the like, occur relatively frequently, such arrangements have a high factor of uncertainty which greatly limits the possible range of application.
Better results with respect to the drift behavior are obtained with the arrangement proposed in German Printed Application No. 2,153,910, wherein the signal voltage charges a capacitor which, together with the operational amplifier, is disconnected from the input circuit in the operating state. The voltage across this capacitor, which is approximately constant for the duration of the measuring operation, is used as the reference value. The disadvantage of this arrangement lies in the fact that the storage capacitor gradually loses its charge through leakage currents and through the load presented by the following circuit, so the measuring operation must be completed after a relatively short time. If the switching period is not foreseeable, the circuit will return to the quiescent state with the discharge of the storage capacitor although the measuring operation has not been completed yet. This results in errors, so it is advisable to use the measuring device only if exclusively short switching times are to be expected. Another disadvantage seems to be that, like in all other conventional circuits, unduly high sensitivity variations or defects in the lenses, if not noticed at the beginning, may simulate a switching state which does not exist.